This invention relates to a fluid pumping system and, more particularly, to a fluid pumping system adapted for use with a natural gas dehydrating system of the type employed at a gas well head to remove water from a well stream composed of a mixture of gas, oil and water.
Examples of such gas dehydrating systems are disclosed in U.S. Pat. Nos. 3,094,574; 3,288,448; and 3,541,763; the disclosures of which are specifically incorporated herein by reference. In general, such systems comprise a separator means for receiving the gas-oil-water mixture from the well head and separating the oil and water liquids from "wet" (water vapor laden) gas; and a water absorber means, which employs a liquid dehydrating agent such as glycol, for removing the water vapor from the wet gas and producing "dry" gas suitable for commercial usage. The glycol is continuously supplied to the absorber means in a "dry" low water vapor pressure condition and is removed from the absorber means in a "wet" high water vapor pressure condition. The wet glycol is continuously removed from the absorber means and circulated through a reboiler means for removing the absorbed water from the glycol to provide a new supply of dry glycol. The glycol reboiler means usually comprises a still column associated with a gas burner for heating the wet glycol to produce hot dry glycol by removing the absorbed water by vaporization. The hot dry glycol passes through a heat exchanger, where the hot dry glycol is cooled and the incoming wet glycol is heated, to a dry glycol storage tank. A glycol passage means is provided to enable passage of wet glycol from the absorber means to the reboiler means and to pump dry glycol from the storage tank to the absorber means.
Prior to the inventions described in our copending U.S. patent application, Ser. No. 277,266, filed June 25, 1981 now U.S. Pat. No. 4,402,652 and our U.S. Pat. No. 4,286,929, the disclosure of which are hereby incorporated herein by reference, motors for glycol pumps of natural gas dehydrating systems were designed to be operated by the energy of natural gas available at the well head due to the relatively high pressures and temperatures thereof. In addition, some prior art pumps used the energy of the wet glycol to drive a single piston pump for the dry glycol as disclosed in U.S. Pat. No. 3,093,122 to Sachnik dated June 11, 1963. The Sachnik pumping unit uses a fluid driven power piston, and a pilot valve driven by the same fluid controls the rate of operation of the master slide valve, which distributes fluid to the piston pump.
One of the problems with such prior pump designs is that the pressure of the gas stream from natural gas wells is highly variable and gas operated pumps often require large amounts of energy. Furthermore, changes in gas pressures during day to day operation have often caused stalling of the pump and interruption of the entire dehydrating system. Since the dehydrating systems are continuously operated at the well site without continuous monitoring by operating personnel, reliable continuous operation of the pump is of critical importance.
Another important performance factor is that the pump be self-regulating to automatically adjust the pumping rate in accordance with available gas pressure and flow rates. In addition, it is highly desirable to use energy sources available at the well site for operation of the pump with maximum efficiency and minimum energy loss.
The present invention provides a new improved glycol pumping system which is operated by an available energy source other than the saleable dry natural gas at the well head; which may be operated at relatively low speeds and pressures without stalling; and which is automatically continuously operable under a wide range of operating conditions.
The pumping system comprises a glycol operated motor-pump section and a gas operated motor-controller section. The glycol motor-pump section comprises a cylinder and a piston reciprocably movable therein which provides a variable volume glycol motor chamber on one side of the piston and a variable volume glycol pump chamber on the opposite side of the piston. The motor chamber is alternately connected to high pressure wet glycol from the absorber and to the reboiler through wet glycol flow control valve means. High pressure wet glycol in the motor chamber drives the piston in one direction during a pumping stroke and is exhausted from the one chamber during a return stroke of the piston. Low pressure dry glycol is drawn into the pump chamber from the dry glycol tank piston during the return stroke and is forced from the pump chamber to the absorber during the piston pumping stroke through suitable check valve means. The gas motor-controller section comprises a cylinder and a piston reciprocably movable therein which provide a pair of variable volume gas chambers on opposite sides of the piston. The glycol motor piston and the gas motor piston are connected to opposite ends of a piston rod which extends between the glycol cylinder and the gas cylinder. Dry gas at relatively high pressure is alternately connected to and exhausted from the gas chambers on opposite sides of the gas piston through gas flow control valve means whereby gas pressure acts on the gas piston to assist movement of the glycol piston during the pumping stroke and to act as the primary motivating force during the return stroke of the glycol piston.
In the presently preferred embodiment, the gas flow control valve means is a reciprocable spool type valve operable between opposite gas intake and exhaust positions relative to the gas chambers by alternate application of gas to opposite ends of the spool type valve controlled by the position of the gas piston in the gas cylinder. In addition, the wet glycol flow control valve means is a reciprocable spool type valve operable between spaced opposite glycol intake and exhaust positions by alternate application and exhaust of gas at opposite ends thereof which is controlled by the gas flow control valve means. Thus intake and exhaust of wet glycol at the glycol motor chamber of the motor-pump section is synchronized with intake and exhaust of gas in the gas chambers of the motor-controller section.
In the illustrative and presently preferred embodiments of the invention, a gas operated piston and a glycol operated piston are concentrically mounted on opposite ends of a piston rod of substantially smaller diameter than the gas or glycol pistons. The gas and glycol pistons may or may not be of the same diameter, depending on the design requirements of a given application. The gas and glycol pistons move axially and are sealed within the bores of separate axially spaced gas and glycol cylinders, respectively. The gas and glycol cylinders are mounted on opposite ends of a centrally located seal plate through which the piston rod extends. A central fluid vent cavity is provided in the seal plate to receive any glycol or gas which may bypass seals mounted in the seal plate which normally prevent leakage of glycol and gas from the cylinders into the central vent cavity in the seal plate. Reciprocation of the gas piston is controlled by a four way gas operated shuttle valve of the spool type. Shifting of the gas spool valve is accomplished by a gas pilot system comprising a gas groove on the periphery of the gas piston which is alternately connected to shift ports at opposite ends of the gas cylinder and passages extending to opposite ends of the gas spool valve. Pump speed is generally controlled by a manual control valve mounted in the gas inlet line. Control of the wet glycol to and from the associated motor cylinder chamber is accomplished by a three-way spool type shuttle valve which is shifted by gas pressure signals from the gas shuttle valve which act through flexible diaphragms onto opposite end portions of glycol shuttle valve to thereby shift the glycol shuttle valve from the wet glycol intake position to the opposite wet glycol exhaust position.